The production of hollow fibers by melt spinning or by wet spinning has been known for many years. The processes described in numerous patents are essentially based on three methods.
In the first method, a molten polymer, for example a polyester, is spun from a jet comprising adjacent arc segments. Synthetic hollow fibers are produced by expanding the molten polymer below the jet and allowing the ends of the arc segments to flow continuously into one another. The second method is based on the use of a cavity needle which has to be placed at the center of the jet orifice, gases or filling media being pumped through the cavity needle. The polymer flows around the needle and the gas fills the cavity in the middle and stabilizes the shape of the hollow filament until the polymer has cooled. Hollow viscose filaments in particular are produced in this way, castor oil, for example, being used as the lumen-filling medium. Finally, in the third method, a fixed pin extends into the jet orifice. This spinning process is generally difficult because the polymer tends to assume a closed form. Although this process is particularly suitable for cross-sectional modifications, air has to be introduced or a vacuum applied at the end of the pin to produce hollow fibers.
In the meantime, hollow fibers and filaments have been used for various applications. For example, they are used for the desalination of seawater, for cleaning liquids and gases, as ion exchangers, for reverse osmosis, dialysis and ultrafiltration (artificial kidneys) and, by virtue of their light weight and high bulk, for luxury clothing. The cleaning of materials, for example, industrial gases, has become particularly important in recent years. Comprehensive articles on the production and significance of synethetic hollow fibers can be found in Encyclopedia of Polymer Science and Technology, 15, (1971), pages 258-272, in Acta Polymerica, 30, (1979), pages 343-347 and in Chemical Engineering, February 1980, pages 54-55.
Hitherto, there has also been no shortage of attempts to produce hollow acrylic fibers by dry spinning from a spinning solution. On account of the problems involved, however, no technical process for the production of hollow acrylic fibers on this principle has ever been successfully developed.
In this context, hollow fibers are understood to be fibers which have a continuous bore extending longitudinally throughout their interior.
Whereas acrylonitrile polymers can be wet-spun relatively easily into hollow fibers by one of the methods described above, this leads to considerable difficulties where dry spinning is used because of a different filament forming mechanism. In wet spinning the filament is formed by coagulation of the spinning solution in an aqueous precipitation bath containing a solvent for polyacrylonitrile, the concentration and temperature of the precipitation bath and additional coagulants, such as aqueous salt solutions, for example, being variable within wide limits. Thus, DE-OS No. 23 46 011, for example, describes the production of hollow acrylic fibers by method 2 for a wet spinning process using aqueous DMF as the precipitation bath whereas DE-OS No. 23 21 460 uses aqueous nitric acid as the precipitation bath, the filament being spun from jets having annular orifices and a liquid being introduced as an internal precipitant into the middle part of the annular orifice.
Any attempt to apply methods 1 to 3 to a dry spinning process is met with considerable difficulties because, where filaments are spun from a spinning solution, some of the solvent has to be evaporated after emerging from the jet orifice to achieve filament formation and hardening. On account of the considerable technical outlay involved and the difficulties in carrying out the process in the production of hollow acrylic fibers by dry spinning from spinning solutions, methods 2 and 3 were not developed any further.
Attempts to dry spin hollow fibers by method 1 from so-called profile jets comprising adjacent arc segments generally result in the formation of only dumbbell-shaped or irregular, bizarre cross-sections containing pockets of air at irregular intervals. If the concentration of polymer solids is increased to obtain the required cavity profile by increasing the structural viscosity, unexpected problems arise. The extent to which the solids content can be increased is limited on account of the gelation, fluidity and handling of spinning solutions of the type in question. For example, an acrylonitrile copolymer of which the chemical composition is 93.6% acrylonitrile, 5.7% methylacrylate and 0.7% sodium methallyl sulphonate and which has a K-value of 81 can only be dissolved in a spinning solvent, such as dimethyl formamide, and spun into filaments up to a solids concentration of at most 32% by weight. If an attempt is made to increase the solids content any further, spinning solutions of this type will gel on cooling at temperatures of only 50.degree. to 80.degree. C., with the result that uninterrupted spinning becomes impossible.
The object of the present invention is to provide a dry spinning process for the production of hollow acrylonitrile fibers, there being numerous potential applications for such hollow fibers and filaments.